1. Field of the Invention
The present invention relates to a method for producing a developing agent for developing an electrostatic charge image or a magnetic latent image in electrophotography, electrostatic printing, magnetic recording and the like.
2. Description of the Related Art
In the related-art production method of an electrophotographic toner, a kneading pulverization method was the main current. In the case of a toner particle to be produced by the kneading pulverization method, in general, its shape was amorphous, and its surface composition was heterogeneous. Though the shape or surface composition of the toner particle subtly may change depending upon pulverization properties of a material to be used or conditions of a pulverization step, it was difficult to intentionally control the shape. Also, the kneading pulverization method was limited in making the particle size small. Mechanical pulverization of a toner go through steps of pulverization, classification and the like. When the particle size became small, the yield was lowered due to a lowering of the efficiency in the classification, and necessary energy increased.
On the other hand, with the diffusion of a digital color system aiming at a high image quality of these days, needs for realizing a small particle size of a toner have increased. The small-sized toner is able to increase a coverage on a medium such as paper at a low consumption amount and is especially advantageous for colorization of electrophotography.
Also, from the viewpoint of enhancing transfer properties and fixability, it has been demanded to precisely control a toner particle regarding the toner shape, particle size distribution and encapsulation and the like. As a production method which meets these demands, the production of a toner by a polymerization method is exemplified.
As the method for producing a toner by a polymerization method, there are an emulsion polymerization coagulation method, a suspension polymerization method, a solution suspension method and the like. Among these methods, in order to meet the foregoing precise control of a toner particle, an emulsion polymerization coagulation method is advantageous as described in JP-A-2-61650 and JP-A-2-259770. This method is a technology for obtaining a targeted particle size by forming a resin fine particle by emulsion polymerization and then performing coagulation. According to this method, though there is a possibility that a particle having a small particle size and having controlled particle shape and particle size distribution is obtained, a step of coagulating a fine particle is essential.
With respect to the coagulation method, it is general to obtain a particle having desired particle size and particle size distribution by using an inorganic metal salt or the like as a coagulating agent and changing the operation temperature and stirring state in the coagulation step. For example, there is a technology for coagulating a dispersed resin particle at a temperature of not higher than a glass transition point thereof as disclosed in Japanese Patent No. 3141783 and JP-A-2005-62887. Alternatively, for example, there is a technology for controlling a power required for stirring as disclosed in Japanese Patent No. 3610543.
In the case where a styrene based resin is used as a binder resin, it is possible to produce a small-sized toner by employing the foregoing emulsion coagulation method. However, in recent color toners, since fixability is required at a lower temperature, a resin having a low glass transition point is suitable, and polyester based resins are frequently used.
However, coagulation with respect to a polyester based resin was studied. As a result, there were obtained only materials in which the viscosity of a solution increases, whereby stirring becomes impossible, or even when coagulation is possible, the amount of coarse particles is remarkably high. Such became more remarkable as the concentration of solids of the resin at the coagulation increased.
It is thought that this is caused due to the matter that the solution exhibits thixotropy because a resin having a terminal functional group as in polyester based resins has a network structure where the functional group is organically coupled.
Also, for example, there is a method for obtaining a atomized binder resin by dissolving a binder resin in an organic solvent, adding dropwise water in the mixture to achieve inverse emulsification and removing the organic solvent as disclosed in JP-A-2002-6550. Though this method is known as a method for atomizing a resin, and therefore, a step of removing the organic solvent is included, the steps became complicated. Also, this method involved a problem that the organic solvent adversely affects the environment.